1. Field of the Invention
The present invention relates to a secondary lithium battery.
2. Description of Related Art
Conventionally, lithium cobalt oxide is the mainstream of cathode active materials used in secondary lithium batteries, and the secondary lithium batteries employing this material are widely used. However, cobalt which is a material of the lithium cobalt oxide is small in output quantity and expensive, and hence alternate materials are being sought.
A lithium manganese oxide having a spinel structure (lithium manganese oxide spinel) which is considered as an alternate material should be further improved in terms that the discharge capacity thereof is not sufficient and Mn is dissolved at high temperature. Also, a lithium nickel oxide which is expected to improve battery capacity should be further improved in terms that the thermal stability thereof at high temperature is low.
Accordingly, a lithium-containing composite oxide which forms polyanion (hereinafter, referred to as a “polyanion cathode active material”) is expected as a novel cathode active material, the polyanion having high thermal stability and being excellent in safety.
Examples of the polyanion cathode active material and the polyanion contained therein include PO43− anion contained in LiMPO4, SO44− anion contained in Li2MSO4, and BO33− anion contained in LiMBO3, etc., where M is any metallic element.
In particular, the most researches are focused on the lithium metal phosphate (olivine) represented by LiMPO4.
Japanese Patent Laid-Open No. 2004-55493 (Patent Literature 1) discloses a cathode using a diene rubber such as styrene-butadiene rubber copolymer (SBR) etc. instead of PVDF as a binder in order to improve flexibility and smoothness and to prevent electrode-peeling and breakage from occurring.
Japanese Patent Laid-Open No. 2003-132893 (Patent Literature 2) discloses a slurry composition for an electrode using a polyvinyl resin excluding fluorine, such as polyolefin, polyvinyl alcohol and polyacrylic acid, and a copolymer thereof as a binder that has a low degree of swelling for an electrolyte solution and that is excellent in binding property.
Each of Japanese Patent Laid-Open Nos. 2004-63300, 2003-257433 and 2002-304997 (Patent Literatures 3 to 5) discloses both a secondary lithium-ion polymer battery that has a first adhesion layer between a positive current collector and a cathode active material layer and a second adhesion layer between a negative current collector and an anode active material layer, each of the first adhesion layer and the second adhesion layer containing a third binder and a conductive material, and a resin composition used in the third binder.